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Probing the dark halo of the Milky Way with GeMS/GSAOI


Principal Investigator: Tobias Fritz, University of Virginia


Program Summary:

The Local Group, the regime in which detailed star-by-star studies can be done, is becoming a testbed for the study of the processes of galaxy formation in general. This is because the constituents of the Local Group span a wide range of parameters such as star formation efficiency, dark matter halo size, and environment. These parameters can be turned into tests of the pressing questions governing both cold dark matter theory and galaxy formation studies, such as the distribution of matter on small scales, and even the impact of reionization. A better estimate of the total Milky Way halo mass is important for many of these questions. Due to the mass anisotropy degeneracy it is not well determined from radial velocities. Current constraints on the shape of the halo are surprising: it is oblate, but misaligned by 90 degrees with the Disk. Proper motions are required in addition to the (generally known) radial velocities to test such halo models. Turning to the baryonic sector, while accretion evidently plays a role in galaxy formation, as attested to by the existence of tidal debris streams, it is not yet excluded that some halo components formed in situ. To disentangle this process in detail, orbits are necessary.

In this program we use GeMS/GSAOI to obtain proper motions, which are missing phase space components, for a variety of tracers in the Milky Way halo, in order to constrain its shape and total mass, as well as the orbital histories of the tracers. We will employ background galaxies as a reference frame to obtain absolute motions. This pioneering study will also produce an astrometric calibration suitable for other uses of GeMS/GSAOI. Using 143 h distributed over three years we will obtain proper motions for a set of dwarf galaxies, globular clusters, and M-giants in the Sagittarius stream, distributed between 20 and 100 kpc in the halo of our Galaxy (see Figure 4 for the target distribution over the sky).


  • Nitya Kallivayalil, University of Virginia
  • Steve Majewski, University of Virginia
  • Gulliermo Damke, University of Virginia
  • Rachael Beaton, University of Virginia
  • Jo Bovy, Institute for Advanced Study
  • Mike Boylan-Kolchin, University of Virginia
  • Ric Davies, Max Institut for extraterrestrial physiscs
  • Rodrigo Carrasco, Gemini Observatory - South
  • Roeland van der Marel, Space Telescope Science Institute
  • Tony Sohn, Space Telescope Science Institute